The reliability of a power transistor is largely determined by thermal considerations. Power dissipation in the junction regions, due to operation of the transistor, generates heat which accelerates failure mechanisms of the transistor thereby shortening its useful life. Useful life of the transistor can be extended by a transistor mounting scheme which provides a heat sink arrangement to conduct heat away from the transistor package and hence reduce junction temperatures. These mounting assemblies must also provide electrical isolation between the body of the transistor package and the apparatus or chassis on which the transistor is mounted.
This mounting arrangement, at present, typically consists of a single washer conforming to the outline of the transistor package body such as the standard TO3 package and is constructed of a material having the necessary electrical and thermal properties. Beryllia, also known as Beryllium oxide (BeO), is a material prossessing both good electrical isolation propeties and good thermal conductivity. A washer made of Beryllia is situated between the transistor and the apparatus to which it is mounted. The mounting is secured by mounting fasteners passing through holes in the transistor package and matching holes in the washer. Silicon grease, a thermally conductive medium or an equivalent heat conductive compound, is inserted between the washer and the transistor package to coat both surfaces and improve thermal conduction between adjacent surfaces that are generally not entirely flat.
It is apparent, therefore, from the foregoing that a successful mounting scheme for power transistors must take into account the thermal properties of the transistor as well as its electrical properties. Any mounting scheme selected must further consider the mechanical properties of the mounting system, cost of materials and ease of installation.